Ceramic tiles are commonly formed by pressing material in powder form, of between 1% and 10% moisture content, within a mould. This forming method is commonly known as dry forming. The soft material is loaded into the mould by known means. After the mould has been closed by punches operated by the pressing members, the powder undergoes initial light pressing, with consequent volume reduction, facilitating powder deaeration. The inital light pressing, also known as a first pressing, is followed by the deaeration stage, during which pressing is interrupted and the could is sometimes reopened to allow the air to escape. The light pressing subjects the powder to a pressure which is about one tenth of the pressing pressure. This is followed by the main pressing to a pressure of about 400 kg/cm2, which ensures perfect powder compaction. The main pressing generally takes place in several successive steps at increasing pressure up to the maximum pressure. The thrust exerted by the upper cross-member of the press is distributed over the total surface of the tiles pressed during each cycle.
It should be noted that each time reference is made to the term xe2x80x9cpressurexe2x80x9d in the text, this unless otherwise specified means the compacting pressure to which the powder is subjected within the forming mould. The largest currently available presses have a capacity (pressing force) of 4000 tonnes, and during each cycle are able to press a surface area of not exceeding 10,000 cm2. Thus, for example, they can operate a die having three impressions of 54 cmxc3x9754 cm.
Tiles of large and ever larger dimensions, having sides exceeding one meter, have not been able to be formed so far by known dry processes because the capacity (pressing force) of the press required to compact the pressure would involves a structure of such dimensions as to be difficult to construct.
Large-dimension tiles having sides of the order of one meter or more are currently manufactured either by extrusion processes or by wet forming processes within hygroscopic moulds similar to those used for sanitary appliances.
Apart from the low cost effectiveness of such processes, the subsequent high-temperature firing of the material creates important problems due to the excessive or poorly distributed moisture contained in the material.
The object of this patent is to achieve dry-forming of ceramic tiles by powder compaction using compacting pressures not strictly related to the press capacity, ie to the maximum pressing force which the press can exert.
The purpose of this is to be able to manufacture, particularly but not exclusively, large-dimension tiles having for example a side dimension of the order of 100 cm using currently available pressing forces, ie presses of currently available capacity.
The present invention is also convenient for manufacturing tiles of usual dimensions using low-capacity presses, which by virtue of the invention are able to exert compaction pressures of up to 500 bar.
The method of the present invention is achieved by dividing the title surface into two or more portions, preferably of equal surface area, and pressing these portions, not simultaneously, but one at a time, in succession. It is immediately apparent that by dividing the surface to be pressed into two portions having the same area. The press capacity is halved, or for equal press capacity the powder compacting pressure is doubled.
To implement the method the mould punch must be divided into adjacent portions, preferably having the same surface area or areas of the same order of magnitude. For example such punch portions can conveniently be concentric.
The pressing cycle according to the present invention comprises the following operations.
The powder is loaded into the mould in a conventional manner, i.e. having expelled the tile the movable carriage grid carries the powder into the mould die.
A cross-member carrying the upper punch divided into portions is then lowered to close the mould. An initial light compaction, or first pressing follows.
The first pressing can be done by moving the various (for example two) portions of the punch as if the punch were in one piece. This is because the compacting pressure required for the first pressing multiplied by the total tile area certainly does not exceed the pressing force which can be exerted by the press.
In certain special cases, the first pressing can also be carried out at pulsating pressure by moving the various punch portions as if it were a one-piece punch or by alternating the pressure of the various parts of the punch.
Considering a punch divided into two portions of about equal surface area, for example concentric, as the maximum pressing force exertable by the press is achieved by pressing simultaneously with the two punch portions, part of the pressing force is applied in succession, for example firstly to the first punch portion, after which the first portion is unloaded and part of the pressing force is transferred to the second portion and so on, applying force increments of force until the entire force is applied firstly to one portion and then to the other.
As a modification, instead of applying force increments of force alternately to one portion and their to the other portion of the punch until the entire press pressing force is attained, the entire press force can be applied from the beginning, firstly to one punch portion and then to the other.
The divided punch can be the upper punch or the lower punch, or a combination of both.